Conventional pressure transducers accommodate the applied force by means separate from those associated with the generation of the related electrical signal output. For example, a known electrically operated pressure transducer relies on the deflection of a precisely machined diaphragm to provide the mechanical strain to activate corresponding changes in strain in a vibrating wire attached or coupled to the diaphragm. The applied force is thus dissipated on the diaphragm and the electrically excited vibrating wire only measures the diaphragm's response to that force. Because what is initially produced is the strain or deformation in the diaphragm, which in turn is translated to strain or deformation in the wire, such transducers commonly encounter problems of nonlinearity over the range of expected measurement values, in addition to other problems in the wire such as long-term wire creep or tension loss, crimp slippage or deformation, and difficult thermal expansion compensation. Moreover, the typical reverse direction in which strain in the vibrating wire is relieved compounds all of the above problems.
FIG. 3 is typical of a well established vibrating wire pressure transducer which consists of four parts. Sensing diaphragm 1 is a thin, precisely machined pressure sensing diaphragm, sealed and rigidly mounted to body 2. The diaphragm has a controlled axial mechanical strain response when subjected to an external pressure different from the internal reference pressure. Ranges may be changed only by changing its geometry, or by changing the flat surface thickness of the diaphragm. Body 2 forms the reference or datum bed across which a musical quality wire 3 is stretched and retained between itself and the center point of the diaphragm 1. Prestressed musical wire 3 is prestressed during assembly to approximately 25% of yield strength and retained between body 2 and diaphragm 1. This prestressed musical wire forms an element of the transducing mechanism in well known fashion by responding to the diaphragm axial strain caused by external pressure application to the diaphragm. Wire tension thereby decreases as the diaphragm is strained inward by increased external pressure. Thus the frequency at which it will mechanically vibrate is a function of diaphragm deflection and, hence applied pressure. Permanent magnet and coil assembly 4 provides a means in a well known manner of both electromagnetically exciting the wire into oscillation and subsequently facilitating the generation of an output electrical signal at the wire's resonant frequency to a remote measuring point via a suitable cable.
This conventional prestressed wire type pressure transducer exhibits one significant advantage over other types of established pressure sensors in that its electrical output signal format is frequency rather than analog magnitude related, and as such is largely insensitive to cable leakage, resistivity changes, etc. As a result of this attribute, the reading of such instruments over several kilometers of cable is possible without downgrading of accuracy. For this reason the vibrating wire type of device is favored in the hostile environment of civil engineering applications, particularly with regard to foundations and earthworks. However, the prestressed vibrating wire device discussed above inherently suffers certain limitations which over the life of the installed, and often irrecoverable transducer can and has raised doubts in users regarding the long-term stability of such units. One of the potential problems relates to long-term creep or loss of tension in the wire, whereby with time the stress level in this prestressed component decreases independent of applied pressure, and thus lowers the wire's resonant frequency.
A prestressed vibrating wire type pressure transducer is also known which employs a bellows with a very high spring rate in series with the prestressed vibrating wire. As with other conventional transducers of this type an increase in pressure applied to the interior of the bellows results in a decrease in tension in the vibrating wire. The spring rate of the bellows is necessarily high in order to maintain the high stress levels in the wire which are required in this type transducer.
Since this type of transducer, as discussed above, senses applied pressure as a relief of tension in the prestressed wire which reduces the resonant vibrating frequency of the wire, and since reduced frequency is usually indicative of increased applied pressure, such a long-term decrease in wire stress, and hence resonant frequency is taken by the user to be an increase in applied pressure, whereas in reality all pressure readings subsequent to tension loss are in error by the unknown extent of this tension loss.
The dimensions of this potential limitation can be illustrated by considering a typical vibrating wire pressure transducer with the following characteristics: EQU Range +0-100 psi EQU Diameter of wire +0.009 inch EQU Length of wire +1.5 inch EQU Frequency output range +4000.times.f.sup.2 /1000 digits
where f.sup.2 /1000 refers to a preferred readout mode wherein f.sup.2 is the square of the vibrational frequency, and f.sup.2 /1000 is a single unit for readout purposes. In such a case, wire prestressing during assembly will stretch it by about 3 thousandths of an inch to achieve the zero applied pressure frequency of about 3000 Hz (9000 f.sup.2 /1000 indicated reading).
As the unit is externally loaded to full range pressure (100 psi) the pretensioned wire stress will reduce as the diaphragm strains to the applied pressure. This equates to approximately 1.2 thousandths of an inch strain over the free wire length, and as such represents the response over the full operating range of the instrument.
In terms of strain therefore, only one hundredth part of 0.0012 inch, or 0.000012 inch component creep, in the diaphragm, body, wire or wire gripping points is necessary to produce an offset in frequency equivalent to 1% of instrument range. In terms of wire stress, only 0.4% of loss in wire prestress is necessary to cause a 1% instrument zero drift. In most civil engineering applications this cannot be detected or quantified after the unit is irretrievably installed for use.
In common with most established types of pressure transducers employing a diaphragm and separate strain measuring elements, the prestressed vibrating wire device design discussed above requires special attention to be paid to the relatively different temperature coefficient factors of the various components, in order to minimize performance changes which occur where the temperature effects on the body 2 and diaphragm 1 do not match the effects on the prestressed wire 3.
Where temperature effects on the body and diaphragm do not match temperature effects on the wire, differential expansion or contraction occurs and causes output frequency changes to occur which cannot be differentiated from real applied pressure variations. This is so even in cases where there are careful design efforts to select the correct proportion of different materials in diaphragm and body parts to match the temperature response over a certain range of temperatures because all components are not necessarily at the same temperature at any given time.
Temperature gradients across the various components, as the influencing temperature changes, will cause significant though transient errors in the magnitude of applied pressure as read. This factor is usually of greatest significance at the time of installation where the instrument is often subjected to changes of temperature environment as placement occurs, and on such occasions the registering of erroneous datum readings before temperature stabilization occurs is not uncommon.
Accordingly it is an object of the invention to measure the magnitude of a pressure by applying it directly, without the intervening modality of diaphragm strain, as completely as practically possible to a vibrating elastic member which is substantially not prestressed and which bears virtually all of the force of such pressure, and in so doing, mechanically provides by its nature, means by which a frequency related, rather than analogue based, electrical output signal is derived directly from the stress in the elastic member.
It is another object of the invention to substantially reduce the prestress by reversing the applied pressure's effect on vibrating elastic member so that stress increases with increasing applied pressure.
It is another object of the invention to accommodate an applied pressure force and generation of a related electrical signal output by one in the same means.
It is a further object of the invention to significantly minimize the effect of long-term creep or tension loss discussed above by employing material in the vibrating elastic member other than hard musical grade wire and by not prestressing the vibrating elastic element or doing so at levels considerably below the 25% of yield figure referred to above.
It is a still further object of the invention to provide a vibrating elastic member configuration which is inherently less sensitive to thermal expansion and to temperature gradient effects than the transducers discussed above.
It is another object of the invention to provide increased reading resolution for full rated pressure range.
It is a further object of the invention to provide an invention with a design which results in uniformity of design for a variety of pressure ranges and in low completed item wastage rates.
These and other objects of the invention are accomplished according to the disclosure of the invention herein described as more fully set forth below.